Evaluation of a Spawning Habitat Enhancement Site for Chinook Salmon in a Regulated California River

Abstract

An evaluation of the effectiveness of a project to enhance spawning habitat for Chinook salmon Oncorhynchus tshawytscha was conducted in the Mokelumne River, a regulated stream in California's Central Valley. Approximately 976 m³ of clean river gravel (25–150 mm) was placed in berm and gravel bar configurations along the 45-m enhancement site. Physical measurements taken before and after gravel placement indicate that the project significantly increased channel water velocities, intergravel permeability, and dissolved oxygen; reduced channel depths; and equilibrated intergravel and ambient river temperatures. These positive benefits remained throughout the 30-month monitoring period. Adult Chinook salmon began spawning at the previously unused site within 2 months after gravel placement and continued to use the site during the three spawning seasons encompassed by the study. Bed material movement was documented by channel bathymetry surveys over two water years. Topographical channel surveys provide a useful tool for monitoring bed material transport and layering redd locations on contour maps. Although its usefulness in restoring salmon populations is poorly understood, gravel enhancement can be an effective means for improving salmon spawning habitat in rivers where upstream dams have effected low gravel recruitment.